With respect to the prior art, an explanation will be given of a device disclosed in Japanese Laid-Open Patent Application No. 188301/1998 (Tokukaihei 10-188301, published on Jul. 21, 1998).
FIG. 17 is an explanatory drawing that shows a combination lens in accordance with the conventional technique.
This application discloses a combination lens barrel which changes the combination of lenses and the gap between the combined lenses so that the spherical aberration of a converged light spot on a recording medium is altered.
As illustrated in FIG. 17, upon converging light on a recording medium 104 through a combination lens 106, a first lens 100 is placed on the side of a light source, not shown, and a second lens 101 is placed on the side closer to the recording medium 104 with respect to the first lens 100. A focus radial actuator (FR actuator) 102 allows the first lens 100 and the second lens 101 to shift in focusing and radial directions. A spherical-aberration correcting actuator 103 drives the second lens 101 to shift in the focusing direction so as to change the gap between the first lens 100 and the second lens 101, thereby making it possible to correct the spherical aberration of a converged light spot formed on a recording layer 105 of a recording medium 104.
Light rays, emitted from a light source (not shown), are directed to a combination lens 106 by an optical part (not shown), and converged onto the recording layer 105 of the recording medium 104.
The reason that the combination lens 106 consisting of a plurality of lenses (first lens 100, second lens 101) is provided is because the numerical aperture of the lens is increased, with the result that the use of only one lens makes it difficult to design and manufacture a light-converging system that can converge light efficiently.
When there is an error in the optical thickness from the surface of the recording medium 104 on the combination lens side to the recording layer 105, a spherical aberration is generated on a light spot converged on the recording layer 105. Here, since the numerical aperture of the lens is great, the amount of generation of the spherical aberration with respect to the error in the optical thickness from the surface on the combination lens side to the recording layer 105 becomes greater as compared with a lens having a low numerical aperture. For this reason, the gap between lenses is changed so as to reduce the generation of spherical aberration and also to provide an arrangement that is suitable for a recording medium having two or more recording layers.
In this case, the optical thickness refers to a thickness determined by a thickness of a light-transmitting body (or a light-transmitting layer) that transmits light and its refractive index; and even in the case when thicknesses (mechanical thicknesses) are different, if the sizes of spherical aberrations of light spots converged through the respective light-transmitting bodies are coincident, the optical thicknesses of them are assumed to be the same.
Moreover, the error in the optical thickness from the surface of the recording medium on the lens side to the recording layer refers to a difference between an optical thickness of a light-transmitting body (or a light-transmitting layer) that has been assumed at the time of the lens designing and an actual optical thickness from the surface of the recording medium on the combination lens side to each recording layer that is obtained at the time of actually recording/reproducing information on/from the recording medium.
With respect to a driving system for changing the gap between lenses, a system referred to as “voice coil motor” has been proposed, in which: an electromagnetic force is generated by allowing a positive or negative current to flow through a coil so as to generate an electromagnetic force, thereby making the second lens 101 to shift in focussing up and down directions by utilizing a thrust generated between magnets and coils (for example, Japanese Laid-Open Patent Application No. 255290/1998 (Tokukaihei 10-255290, published on Sep. 25, 1998).
Such a spherical-aberration correcting mechanism, which changes the gap between lenses, makes it possible to properly adjust the amount of shift of the second lens 101, that is, the gap between the first lens 100 and the second lens 101, so that it becomes possible to correct the spherical aberration generated due to an error in the optical thickness from the recording medium on the combination lens side to the recording layer.
Next, an explanation will be given by exemplifying an invention disclosed in Japanese Laid-Open Patent Application No. 266511/1993 (Tokukaihei 5-266511, published on Oct. 15, 1993).
In FIG. 18, a plano-concave lens 107 and a plane-convex lens 108 are placed between an objective lens 109 and a light source (not shown), and the plano-concave lens 107 is shifted in the light axis direction in accordance with the optical thickness of the optical recording medium (corresponding to “the thickness of the protective layer” in Japanese Laid-Open Patent Application No. 266511/1993 (Tokukaihei 5-266511)) so as to correct spherical aberration.
The light, transmitted through the plane-convex lens 108, is converged on a recording layer 110a of an optical recording medium 110 by an objective lens 109 constituted by a plurality of lenses.
In this case, different from the aforementioned example, the spherical aberration is corrected not by the objective lens 109 constituted by a plurality of lenses, but by changing the lens gap of the lenses (plano-concave lens 107 and plane-convex lens 108) that are placed between the objective lens 109 and the light source.
Moreover, with respect to such a lens driving system, those driven by gears have been disclosed. In other words, a mesh section 111 having a concavo-convex shape formed on the peripheral face of the plano-concave lens 107 and a gear 112 rotatably secured to a shaft are engaged with each other, and the plano-concave lens 107 is driven in the light axis direction by rotating the gear 112.
Such spherical-aberration correcting mechanisms usually have a system driven electrically; and as compared with the use of a lens having a low numerical aperture, the use of a lens having a high numerical aperture needs high power consumption so as to drive the spherical-aberration correcting mechanism, with the result that a problem of wasteful power consumption arises. Moreover, in the case of the arrangement having a spherical-aberration correcting actuator built in a combination lens barrel, as in the case of the voice coil motor disclosed in Japanese Laid-Open Patent Application No. 255290/1998 (Tokukaihei 10-255290), upon application of a current to a coil, the coil is heated, with the result that parts in the lens barrel are subjected to thermal expansion; this causes variations in the gap, tilt and di-center between the first and second lenses, resulting in a failure in providing an appropriate converged light spot.